Ancient Audoiphile

D-75A and Green Leds ?

2 posts in this topic

I recently purchased three D-75A amplifiers and I am curious about just how much power is being sent to the speakers when the “Green” signal present turns on? This is something that is not published in the owner's manual.

I am using one in dual mode to feed a pair of JBL 2405H UHF drivers (8 ohms) and two in bridged mono mode to feed a pair of JBL 2446H High Frequency Compression Drivers (8 ohms). So, I would like to know the power output for both modes.

The only thing that I can say about this combination of Amps and Drivers is that it gets deafeningly loud, long before the “Green” LEDs start to flash dimmly. Wow, does it sound sweet...

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The Green LEDs on the front of the D75 are not power meters they are a Signal Presence Indication (SPI). The SPI indicators will light anytime the output voltage reaches about 1 volt peak to peak or above.

Since the SPI LEds are monitoring the output any adjustment of the front panel input attenuators will vary the SPI LEDs illumination.

The impedance, of a speaker, changes with frequency thus the load presented to the amplifier, by the speaker, will continually vary dependant on the input signal source. To measure the output power you would need either a Power Meter or a voltage and current meter to be able to measure the amplifiers output power.

We could “roughly” estimate the power, using Ohms Law (Voltage squared / Resistance). An 8-ohm nominally rated speaker with a one volt output from the amplifier would be getting “about” an 1/8th watt. A 4–ohm speaker would be getting about a ¼ watt.

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